BIO203 - Chapter 5 Summary - CHAPTER 5 SUMMARY Evolutionary...

CHAPTER 5 SUMMARY Evolutionary Considerations and Invertebrate Nervous Systems •The nervous system is one of the two control systems of the body, the other being the endocrine system. In general, the nervous system coordinates rapid responses, whereas the endocrine system regulates activities that require duration rather than speed. •Nervous systems have become progressively more complex during evolution, evolving from simple reflex arcs to centralized brains with distributed, hierarchical regulation. Reflex arcs (found in most animals) consist of sensors, neurons and effectors that quickly detect common disturbances and activate corrective responses. Higher levels of control include anticipation activation of reflexes and memory-enhanced responses. •Sponges have no nerves, but some respond to stimuli using electrical signals. True nervous systems probably first arose as nerve nets in cnidarians. Nerve nets have no central integrator and control relatively simple behaviors. •Simple ganglia (clusters of neurons) and nerve rings evolved for more complex behavior, appearing first in swimming cnidaria. •A true central nervous system or CNS first evolved with bilateral symmetry, as found in flatworms. •Complex nervous systems with distributed ganglia are characteristic of higher invertebrates, such as molluscs and arthropods. True brains evolved at the anterior end of advanced animals such as arthropods, cephalopods and vertebrates. Cephalopod brains are the most advanced among invertebrates, having complex structures supporting complex behaviors including learning. •Vertebrate brain size varies up to 30-fold for a given body size. Relative brain size is hypothesized to result from several factors, including a trade-off between expensive tissue (brains, digestive systems).

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•Some nervous systems can exhibit plasticity, changing in structure with experience. The Vertebrate Nervous System v The nervous system consists of the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system, which includes the nerve fibers carrying information to (afferent division) and from (efferent division) the CNS. HThree classes of neurons—afferent neurons, efferent neurons, and interneurons—compose the

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